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About short circuit currents and its definition

 

About short circuit currents and its definitionKnowing the magnitude of the short circuit current is essential to ensure fire safety. It is obvious that if the measured short-circuit current is less than the setting current of the maximum protection of the machine or 4 times the fuse current rating, then the response time (fuse blowout) will be longer, and this, in turn, can lead to excessive heating of the wires and their a fire.

How to determine this current? There are special techniques and special instruments for this. Here we consider the question of how to do this, having only a multimeter or even a voltmeter. Obviously, this method does not have very high accuracy, but still sufficient to detect a mismatch between the overcurrent protection and the magnitude of this current.

How to do it at home? It is necessary to take a sufficiently powerful receiver, for example, an electric kettle or an iron. It would be nice to have a tee. We connect our consumer and a voltmeter or multimeter in the voltage measurement mode to the tee. We record the steady-state voltage value (U1). We turn off the consumer, and record the voltage value without load (U2). Next we make the calculation. It is necessary to divide the power of your consumer (P) by the difference of the measured voltages.

Ic.c. (1) = P / (U2 - U1)

We show an example. Kettle 2 kW. The first metering is 215 V, the second metering is 230 V. According to the calculation, it turns out to be 133.3 A. If it is, for example, a VA47-29 automatic machine with characteristic C, then its setting will be from 80 to 160 amperes. Therefore, it is possible that this machine will work with a delay. According to the characteristics of the machine, you can determine that the response time can be up to 5 seconds.

Protective characteristic of a circuit breaker

Which is basically dangerous. What to do? It is necessary to increase the value of the short circuit current. You can increase this current by installing a more powerful transformer at the substation or increase the cross-section of the wires of the supply line.

More real way. Check the value of this current directly on the machine. If this current is large enough (more than 200 A in this case), check the connections (contacts) to the place of measurement or replace the wiring within the apartment (house).

See this topic: Calculation of short circuit currents for beginner electricians

See also at bgv.electricianexp.com:

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  • Why do phase-zero loop resistance measurements be performed by professionals and not ...
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    Comments:

    # 1 wrote: | [quote]

     
     

    or 4 times the fuse current rating

    ... three times the rated current of the fuse fuse; "

    Where does the number "4" come from?

    Further. According to the text, it is not clear what rating we take to protect the circuit.

    submachine gun VA47-29 with characteristic C

    nothing tells us about the rated current of the machine. Only knowing the type of curve and the values ​​in 80 and 160A, we understand that this is a 16A machine.

    And the last one. Such short-circuit current will not pass according to the requirements of PTEEP adj. 3 p. 28.4

    In the event of a short to ground or a neutral conductor, the single-phase short-circuit current must be at least: ... 1,1InomN for circuit breakers with instantaneous trip

    Another option, if replacing the wiring or pulling contact connections is not possible, replace it with a device of the same rating, but with a “B” curve, if consumers allow it.